1. Field of the Invention
The present invention relates to a fuel cell system which includes a fuel cell having a plurality of power generation units arranged in a same plane, and electrically connected in series. Each of the power generation units includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Typically, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. Each of the electrodes comprises an electrode catalyst layer of noble metal supported on a carbon base material. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is a power generation unit interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of a fuel cell (unit cell) for generating electricity. A predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
If the fuel cell stack is mounted on a vehicle, hundreds of unit cells are electrically connected together in series for generating the desired level of voltage. In the event of a failure or a malfunction of any of the unit cells, the power generation performance of the entire fuel cell stack may be affected undesirably. In order to prevent the problem, typically, purging operation is performed by supplying the fuel gas to the anode while the power generation is stopped, or performed at a low level.
However, it is not efficient to stop the power generation of all the unit cells when one of the unit cells has a failure or the like. In particular, in the automobile application, hundreds of unit cells are used to form the fuel cell stack. Therefore, every unit cell needs to have high reliability, and a dedicated diluter for diluting the fuel gas used in the purging operation is required. Such equipment would raise the cost of the fuel cell system.
In an attempt to address the problem, for example, Japanese Laid-Open Patent Publication No. 2002-56855 discloses a flat fuel cell in which a plurality of unit cells are arranged in the same plane in a single row, or a plurality of rows, and are electrically connected in series.
FIG. 18 shows the flat fuel cell. The flat fuel cell includes unit cells 4a through 4d. Air electrodes (cathodes) 2a through 2d and fuel electrodes (anodes) 3a through 3d are provided on both sides of electrolyte layers 1a through 1d. The same electrodes are arranged on the same side of the electrolyte layers 1a through 1d, i.e., the cathodes 2a through 2d are arranged on one side of the electrolyte layers 1a through 1d, and the anodes 3a through 3d are arranged on the other side of the electrolyte layers 1a through 1d. Conductive Z-like connection plates 5a through 5d connect the unit cells 4a through 4d together in series.
Specifically, the conductive Z-like connection plate 5a connects the cathode 2a of the unit cell 4a and the anode 3b of the unit cell 4b, the conductive Z-like connection plate 5b connects the cathode 2b of the unit cell 4b and the anode 3c of the unit cell 4c, and the conductive Z-like connection plate 5c connects the cathode 2c of the unit cell 4c and the anode 3d of the unit cell 4d. The anode 3a of the unit cell 4a is connected to a terminal 6a, and the cathode 2d of the unit cell 3d is connected to a terminal 6b. 
Thus, the desired level of voltage can be generated by one flat fuel cell, and a plurality of the flat fuel cells may be stacked together to output an electrical current having a current value depending on the number of the flat fuel cells.
However, since a plurality of the flat fuel cells are electrically connected together, a large potential difference may occur undesirably between the fuel cells in the event of a failure in any of the fuel cells. Consequently, a reverse voltage may be applied to the flat fuel cell of the low voltage level, and the flat fuel cell would be damaged.
According to the disclosed prior art technique, the dedicated Z-like connection plates 5a through 5c are required for connecting the unit cells 4a through 4d electrically in series. A large number of Z-like connection plates are required when many unit cells are used in the fuel cell. The number of components of the fuel cell is large. Further, it is difficult to maintain the reliability of the seal structure or the like. Moreover, the thickness of the fuel cell in the direction indicated by an arrow T is large. Thus, the overall size of the fuel cell is not small.